1. Field of the Invention
The present invention relates to a portable optical disk apparatus for recording or reproducing information signals on an optical disk, which employs an objective lens having an NA (numerical aperture) of 0.8 or higher.
2. Related Background Art
Optical disk apparatuses have been commercialized, which record information signals by focusing a laser beam in a fine spot by an objective lens onto an information signal recording layer of an optical disk or reproducing the recorded information signals by the reflected light. Lately, it has become possible to increase the NA of the objective lens of the optical disk apparatus to be 0.08 or higher, as the results of improvement in the design and production technique of the objective lens. Such a high-NA objective lens enables the formation of a finer light spot, to improve the resolution in information recording and reproduction, and to increase the capacity of the optical disk.
Conventionally, a typical optical disk has an information signal recording layer formed on a substrate plate having a thickness of 0.6-1.2 mm, and which is composed of a transparent resin material. The optical disk apparatus projects a laser beam through the substrate plate onto the information signal recording layer. However, an objective lens of a higher NA has a shorter focus length, requiring a shorter distance between the objective lens and the information signal recording layer. This makes it difficult to focus a laser beam through the substrate plate having a thickness of 0.6-1.2 mm on the information signal recording layer. To solve this problem, certain methods have been disclosed. In one method, to meet the increase of the NA of the objective lens, the information signal recording layer is covered on the face reverse to the substrate plate side with a transparent cover layer thinner than the substrate plate, and the laser beam is projected through the cover layer, and not through the substrate plate, as disclosed in Japanese Patent Application Laid-Open Nos. 2003-91833, 2004-30835, and so forth.
FIG. 4 illustrates schematically a constitution of such an optical disk apparatus of the prior art. In FIG. 4, reference numeral 1 denotes an optical disk, reference numeral 2 denotes an optical head, and reference numeral 3 denotes a spindle motor to rotate the optical disk. Optical head 2 is constituted of laser light source 6, collimator lens 7, beam splitter 8, condenser lens 9, photodetector 10, objective lens 20, and actuator 11 for controlling the focusing and tracking with the objective lens 20.
Optical disk 1 is constituted of substrate plate 12, information signal recording layer 13, formed on substrate plate 12 from a phase changeable material capable of changing the reversible phase state, and cover layer 14 having a thickness of about 0.1 mm, which is composed of a transparent resin material. On information signal recording layer 13, recording tracks are formed in spiral or concentric circles. The information signal recording layer 13 may be formed from a magnetooptical material, or a metallic reflecting film having pits (concave areas) formed thereon. Optical head 2 is placed to face cover layer 14 of optical disk 1.
FIG. 5 shows a constitution of actuator 11. Actuator 11 comprises immovable part 15 and movable part 16. Immovable part 15 is constituted of permanent magnets 17a, 17b, yoke 18, and supporting stage 19. Movable part 16 is constituted of objective lens 20, focusing coil 21, tracking coil 22, and lens-holding member 23 for holding the above articles. Elastic supporting members 24a, 24b, 24c, 24d are in a linear shape, and are elastic and highly electroconductive. The supporting members are fixed at the respective ends to supporting stage 19, and hold, at other ends, movable part 16 to be movable freely in vertical and radial directions relative to optical disk 1. Elastic supporting members 24a, 24b, 24c, 24d are also connected electrically to focusing coil 21 and tracking coil 22 on lens-holding member 23.
The optical disk apparatus, as shown in FIG. 4, has an error signal generating circuit 4 and a control circuit 5 for control of focusing and tracking by actuator 11. Control circuit 5 applies electrical control current through elastic supporting members 24a, 24b, 24c, 24d to focusing coil 21 and tracking coil 22.
In recording the information signals, first, optical disk 1 is rotated by spindle motor 3. With the optical disk kept rotated, a laser beam pulse-modulated in accordance with information signals and emitted from laser light source 6 is made parallel by collimator lens 7, allowed to pass beam splitter 8, and focused in a fine spot by objective lens 20 through cover layer 14 of optical disk 1 on information signal recording layer 13.
The information signal recording layer of optical disk 1 is heated and cooled repeatedly by projection of a pulse-modulated laser beam to form recording marks by a phase change to an amorphous phase or a crystal phase by the heating-cooling process.
In reproduction of the recorded information signals, optical disk 1 is rotated similarly by spindle motor 3. With the optical disk being kept rotated, a constant intensity of a laser beam emitted from laser light source 6 is focused in a fine spot through cover layer 14 on information signal recording layer 13. The intensity of the light reflected from information signal recording layer 13 varies in accordance with the recorded marks, whereby the information signal is reproduced.
During the operation of recording and reproducing of the information signals, the light beam reflected by optical disk 1 is further reflected by beam splitter 8, condensed by condenser lens 9, and detected by photodetector 10. Photodetector 10 has a light-receiving face divided into plural sections. Error signal generating circuit 4 generates focus error signals and tracking error signals according to the detected signals at the respective sections of the light-receiving face.
Control circuit 5 applies a control current, based on the focus error signal and the tracking error signal, through elastic supporting members 24a, 24b, 24c, 24d to focusing coil 21 and tracking coil 22. Actuator 11 drives movable part 16 to move perpendicularly toward or away from optical disk 1, or in the radial directions crossing the recording track, by an electromagnetic force generated by an interaction of the control current and the magnetic fluxes of permanent magnets 17a, 17b. 
In such a manner, the focusing is controlled to focus precisely the light spot on the recording track to offset perpendicular displacement by axial deflection of a surface of optical disk 1, or a like cause, and the tracking is controlled to scan the recording track by offsetting radial run-out of the track center in the radial direction.
In the above optical disk apparatus, since the gap (working distance) between objective lens 20 and the surface of cover layer 14 is as small as 0.2-0.6 mm, a stopper, or a like mechanism, cannot be provided at a position in the gap, to limit the displacement of movable part 16, to prevent undesired contact of objective lens 20 with optical disk 1. Naturally, the actuator normally working for the control will keep constant the gap between objective lens 20 and the surface of cover layer 14. However, the actuator may fail in the control, owing to an abrupt impact, vibration, or a like cause, or an adverse effect of dirt or a scratch on optical disk 1 so that a normal error signal will not be generated.
In such an abnormal working state, movable part 16 is excessively displaced, inevitably to come into contact with optical disk 1. Even when the contact occurs, at least objective lens 20 can be protected by providing a protrusion higher than objective lens 20 at a portion of lens-holding member 23 facing optical disk 1 and allowing the protrusion to touch optical disk 1.
However, even in a non-working state, especially, with the power source turned off, when the optical disk apparatus is placed with optical disk 1 held horizontally, and with optical head 2 held above the disk, elastic supporting members 24a, 24b, 24c, 24d may be bent by gravity acting on movable part 16, to cause movable part 16 to contact with optical disk 1. This contact state can sometimes be kept for a long time if the optical disk apparatus is left standing.
Usually, lens-holding member 23 and cover layer 14 of optical disk 1 are made from different kinds of resins. The different kinds of resins, which are kept in contact with each other under pressure, are liable to undergo not only usual environmental deterioration, but also, a physical change, such as local deformation and a chemical change, such as chemical deterioration. In particular, the protecting member should have essential properties for productivity (formability) and mechanical properties, such as rigidity, and the cover layer should have optical properties and physical properties, such as hardness. Therefore, physical and chemical durability cannot readily be given additional to the protecting member or the cover layer. The deterioration of optical properties of cover layer 14 caused by deformation or deterioration of the contact portion can presumably prevent normal recording and reproduction of the information signals disadvantageously.
In recent years, optical disk apparatuses are coming to be used as portable apparatuses for recording and reproducing voices, still-pictures, animated pictures, and so forth. The portable apparatuses are stored arbitrarily in various manners by users, and stored in many cases with the optical disk being kept inserted in the apparatus. Accordingly, the optical disk apparatus having the objective lens of a high NA will inevitably encounter the above-mentioned problems.